How to detect the net flow for any given ip ?

[Bo Yang <struggle@xxxxxxxxxxxxxxxxxx>]

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Hello,everybody
    I am a newbie for linux firewall , and have look at some document
for iptablesand netfilter .
    In my school , we must pay for 20$ for 2G web flow , and the
gateway in our school can tell us how much flow we use in a month !

    I am wondering how linux gateway can recode the web flow for any
given ip ?
    Can iptables do such things for me ?

    Thanks in advance !

                                                          Bo
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